Reset PIXEL_UNPACK_BUFFER at texture manager initialization time

gpu/command_buffer/service/texture_manager.cc

Index: gpu/command_buffer/service/texture_manager.cc
diff --git a/gpu/command_buffer/service/texture_manager.cc b/gpu/command_buffer/service/texture_manager.cc
index 8899aaee3edf69bcbeb6604863f993b21afe53bc..67615bf25d6a3a39a8ca9772d174c0e343a8954c 100644
--- a/gpu/command_buffer/service/texture_manager.cc
+++ b/gpu/command_buffer/service/texture_manager.cc
@@ -1781,6 +1781,11 @@ TextureManager::TextureManager(MemoryTracker* memory_tracker,
 }
 
 bool TextureManager::Initialize() {
+  // Reset PIXEL_UNPACK_BUFFER to avoid unrelated GL error on some GL drivers.
+  if (feature_info_->gl_version_info().is_es3_capable) {

[qiankun, 2016/09/09 09:52:48]

Using this other than feature_info_->IsES3Capable() here because it's difficult
to check feature_info_->IsES3Capable() in gpu_unittests when implementing
SetupTextureManagerInitExpectations in test_helper.cc. I took whole day but
didn't find a good solution. If you have better method, please tell me.  Thanks.

[Zhenyao Mo, 2016/09/09 15:17:08]

You can use feature_info_->IsES3Capable() here and use
gl_version_info.is_es3_capable in the test.  There are some slight difference
between the two, but as far as it doesn't affect the tests, I feel it's no big
deal.

Or is this difference causing tests to fail?

[qiankun, 2016/09/09 18:16:26]

For some test cases, if --enable-unsafe-es3-apis is set,
gl_version_info.is_es3_capable and feature_info_->IsES3Capable() are different.
This makes EXPECT_CALL check fail.

+    glBindBuffer(GL_PIXEL_UNPACK_BUFFER, 0);
+  }
+
   // TODO(gman): The default textures have to be real textures, not the 0
   // texture because we simulate non shared resources on top of shared
   // resources and all contexts that share resource share the same default